Is it a percentage related to the partition size? Or, is it a hard-fixed number?
What if it's a non-system partition? Why would it matter to Windows if it isn't installed into it?
In my case, I run Windows 8.1
I'm not sure about Win8.1 but Windows XP and 7 notify the user when
of free space is left
Disk space checks
I couldn't find any official articles regarding Windows 8.x, but I assume things aren't any different than Windows 7. Apparently there's no way to tweak the threshold values or the polling interval, and you can't exclude specific disks/partitions either.